Gas purification



F. D. SCHREIBER GAS PURIFICATION Filed June 8, 1958 Patented Oct. 29, 1940 UNITED STATES PATENT orrlcs 2,219,113 ons rnnmcA'rToN Fredrick n. schreiber, Domani, n. Application .Tune s, 193s, semi Nanas 5 Claims.

This invention or discovery relates to gas purification; and it comprisesa method of purifying fuel gases, such as coke oven gas and the like, wherein the acidic gaseous constituents such as I H28, HCN and CO: are removed from a stream of the fuel gas andare then separated and utilized, advantageously by converting the HzS into sulfuric acid and converting the HCN into a salable cyanide: and it comprises a new and useful coml. bination of steps for effecting this result, and apparatus for use therein; all as more fully hereinafter set, forth and claimed.

Various methods have been 'proposed in the past for purifying coke oven gas andthe like u to eliminate the-acidic constituents, and especially the. H28, therefrom.v have met with considerable, commercial success.

fying gases, including refinery gas and natural I gas as well as coke oven gas, have-found commercial favor; and these processes have the outstanding advantage that HzS is recoverable in concentrated form, so. that it may be easily burned to sulfur dioxide and .used as' a raw mail terial in the production of sulfiu'ic acid. This is move H28 from gases, quantities of sulfuric acid. A process making' it I possible' to convert the heretofore undesirable HsS into useful sulfuric acid is therefore highly advantageous. Useful embodiments of the hot 1,971,798, 2,028,124, and 2,028,125, for example. I Various methods have also been proposed heretofore for recovering another acidic constituent, the cyanogen or HCN, which is found especially ln coke oven gas. Some of these methodsvhave been found commercially satisfactory, and have U been used in conjunction with various methods of removing the other acidic constituents, and especially the HsS, from coke oven gas and the like. To the best of my knowledge, however, no commercially acceptable method has heretofore been u proposed for recovering HCN from coke oven gas,

aA hot actiilcation process, such as the sodium phenolate process. An object achieved by my u present invention is to remove the HxB and HCN from coke oven gas,- for example, by a hot actincation process such as 'the sodium phenolate process, and to economically recover the HzS and HCN separately, in salable or usable form; One u of the advantages of my invention is that this resome of these methodi;l coke ovens is thus recovered in the Recently, the hot actiiication4 processes of puriy also consume substantial' actification process are described in U. B. Patents etc.; simultaneously with the'rex'noval of HsS byy suit is achieved in a simple and economical manner.

Inaccordan'ce with my'invention as applied to coke oven gas, the gasfrom the coke oven battery is first passed throughprimary coolers and sepa- 5 rators for pitch and tar in the usual manner. and is then passed through a sulfuric acid saturator, to remove the ammonia contained therein. Generally, vapors from the ammonia still, containing the ammonia and the acid gases (including H28 and HCN) contained in the various condensates produced during the treatment of the gas, are 'combined withthe gas stream entering the saturator. Substantially all of the ammonia formed or liberated by carbonizing coal in the. saturator inl the form of ammonium sulfate, which is withdrawn from time to time or continuously for sale, in known manner. v

' The substantially ammonia-free gas ordinarily leaves the saturato'r at a. temperature on the order of 50? to 60 CK. and this gas is passed through a cooler, often designated a final cooler, in which its temperature is reduced to about 20 to 30 C. [n this cooling stage, I nd it advantageous to employ direct cooling with water maintained at the desired temperature, as the condensate formed from the gas during cooling then combines with thev recirculated water. The excess water in the system is discharged from the cooling system to the ammonia still or the ammonia liquor storage tank, from which it is sent to the still. In either case, any ammonia or acidic constituents such as H2B and HCN contained in the liquor are returned to the gas stream, and ultimately recovered.l

, The water circulated through the final cooler system is cooled in an indirect type of cooler suitably connected to the line returning water to the cooling tower, and maintains this water at the desired temperature. I ilnd it advantageous to use an indirect cooler for maintaining the water temperature vrather than an evaporative cooler, in order to prevent loss of HzS or HCN from the system. Because of this precaution, substantially the entire H18 'and HCN contents of the gas are ultimately recovered.

The cooled gas leaving the nal cooler at a temperature of the order of 20 to 30 C. enters an absorber constituting part of a hot actiilcation gas purifying system, which preferably employs sodium phenolate as the-absorbing solution. This absorber is advantageously of the two-section type, with actifled solution supplied Yat .an intermediate point above the first section 5l and more highly actiiied\solution supplied to the top of the absorber above the second section. The absorbing solution is cooled before it is supplied to the absorber, and at the temperature of the solution and the gas, the HzS and other acidic gases. including HCN and CO2, are absorbed, liberating phenol from the phenolate. Most of this absorption takes place in the first or lower section of the absorber. Some absorption of the acid gases takes place in the second section of the' absorber, but the principal function of the second or upper section is to prevent loss of phenol liberated in the lower section. 'I'he solution supplied to this second section is advantageously so highly actiiied that it is not saturated with respect to phenol, and will therefore have a capacity for picking up any phenol carried forward by the gas stream. f

The gas leaving this absorber is purified with respect to acidic constituents as well as ammonia, and may be discharged to the gas holder or distributing system, or passed through the light oil scrubbers if benzol recovery is to be practiced.

The highly actied absorbing solution of sodium phenolate supplied to the upper section of the absorber passes downwardly into the first or lower section of the absorber, where it assists in the absorption of acid constituents from a iurl ther quantity of gas. All of the solution containing the absorbed acid constituents and the phenol liberated by their absorption is then passed through heat exchangers into the top of the actifier. There the acid gases are driven out by heat, which is ordinarily supplied by steam coils adapted to heat the solution. To effect this acticatlon, it is necessary to heat the solution substantially to the boiling point. 'Ihis theoretically causes the phenol and its homologues, such as the cresols, to attain an acidity higher than that of the previously absorbed HaS, CO2 and HCN, thus reversing the condition prevailing in the absorber. The absorbed acid gases are therefore liberated by reaction of their salts with the phenols. For example, sodium-hydrogen sulfide reacts with phenol at this temperature to liberate Has, reforming sodium phenolate.

The solution being actifled passes downwardly through the actiiier tower, where its actification is continuously improved by the progressive high temperature reaction. All of the solution may be passed down through the tower and recirculated through heat exchangers and coolers to a single step absorber. if desired.' Ordinarily, however, it is advantageous to withdraw a substantial proportion of the solution at an intermediate section of the actifler, and return this solution through suitable cooling devices to the rst or lower section of the absorber. This solution is sulclently actied to absorb most of the acidic gases from the stream of fuel gas. I'he remainder of the solution is allowed to pass on down through the 'lower section of the actifier tower, where it becomes very highly actifled due to the longer boiling. In fact, it is desirable to actify this part of the solution so highly that it poverished in phenol, and is thus phenol in the second sorber.

If desired, actiers and absorbers having more than two sections in each may be employed, but this greater complication is seldom, if ever, justified by the improvement in results obtained in commercial scale operation.

The HzS, CO2 and HCN liberated during actification pass out of the actiier, and are usually acable to pick up or upper portion of the abis slightly imcompanied by water vapor and a certain amount of phenol. It is therefore advantageous to pass these eiiluent vapors through a dephlegmator maintained at such a temperature that substantially all of the water and phenol are condensed and returned to the actier. The gases leaving the dephlegmator then consist of about 70 per cent HzS, 16 per cent CO2, and 13 per cent HCN, with ordinary operation and with the coals now generally employed for coking. In a plant producing 20,000,000 cu. ft. of coke oven gas per day, there is discharged from the actifler dephlegmator between 9,000 and 10,000 cu. ft. of these mixed acid gases per hour.

My next step is to separate the HCN, which I desire to recover, from the other acid gases, and I'find that this is conveniently effected by compressing the gases and scrubbing them with cold water. scrubbing with cold water under pressure eifects a preferential absorption of the HCN, and conditions of temperature, pressure and ow of water and gas may be readily adjusted so that all of the HCN is absorbed with relatively small amounts of HzS and CO2. The gases which are not absorbed are discharged from this pressure absorber, and may be forwarded to an acid plant where the HzS is burned to S02 and converted into sulfuric acid in known manner. For example, the contact process may be satisfactorily employed with this gas.

'Ihe aqueous solution produced in the pressure absorber contains all of the HCN originally present in the coke oven gas, and some of the HzS and CO2. This solutionA is pumped to another tower, where it is scrubbed with a counteriiow of purified gas Withdrawn from the discharge line of the phenolate absorber, or from any other convenient source. A veryy small amount/'of this gas is sumcient, when properly contacted with the HCN solution, to eliminate all the I-IzS and CO2 from the solution. The purified gas containing this removed HQS and CO2, and usually containing a small amount of HCN, is returned to the inlet of the phenolate absorber, so that none of these acid gases is lost from the system. If desired, a part of the HCN solution discharged from the pressure absorber may be recirculated through part or all of this absorber to increase its concentration before scrubbing it with the purified coke oven gas. Y

After scrubbing with the purified coke oven gas, the aqueous solution contains substantially only HCN, which is liberated in any convenient manner. One method which I have found to be especially simple and effective is to slightly acidify the solution with a stronger acid, such as H2804, while maintaining the solution under a vacuum. This may be advantageously eiected by discharging the solution to a closed tower or vat, to which a small amount of H2SO4 is supplied while agitating the solution, and maintaining it under a vacuum by connecting it with the intake of a vacuum pump. This completely removes the HCN from the solution, and the dilute acidiiied water may then be discharged to waste or for utilization.

'I'he gaseous HCN withdrawn by the vacuum pump from the solution is readily converted into any of several cyanogen compounds which nd a ready market. If sodium cyanide, for example, is to be produced, the HCN gas need only be bubbled through a suitable solution of caustic soda. This may be done in any suitable device, but I iind it convenient to employ a closed tower through which the caustic solution and the gaseous HCN are passed in counteriiow. The outlet :amavis from the vacuum pump may, for example, discharge into the bottom of such a tower, and any .gas unabsorbed during a single passage through the tower may be returned to the intake of the pump through a suitable conduit. The solution may also be recirculated through the tower if desired, to complete the conversion of NaOH to NaCN. The NaCN solution is continuously removed from the tower or from a circulation sump, and may be further concentrated by evaporation, or otherwise worked up if desired.

In order that my invention may be more readily understood and appreciated, it will now be described briefly with a" reference to the accompanying drawing, which is a flow-sheetsetting forth the essential parts of the process. In the now-sheet, 2 indicates "a saturator of conventional type,to which sulfuric acid is supplied through an inlet 4.v Coke oven gas or the like is introduced/into the saturator through a pipe 6, into which the ammonia still vapors are ordinarily discharged from a pipe 8. The ammonia in the gas reacts with the HzSC/)i to form ammonium sulfate, which is withdrawn from the saturator either continuously or from time to/f time through an outlet I0. i

The ammonia-free gas is discharged from the saturator through a pipe l2, from whichit passes into the bottom of a cooler |4, 'containing suitable packing lli, which facilitates contact between the gas and the cooling water. After cooling the gas, water containing constituents remov`ed therefrom and condensate formed by cooling is discharged from the bottom ofthe cooler through a pipe I8to a sump 20. 1As the amount of liquid in the system builds up, due to condensation from the gas, excess liquid is withdrawn through an overflow line 22, and sent to the ammonia still or Athe ammonia liquor storage tank (not shown). Cooling water is Withdrawn from the sump 20 by a pump 24, and returned through a cooler 26 of the indirect type, and pipe 28, to the top of the final cooler I4, through which it flows downwardly, cooling a further quantity of gas.' The indirect cooler 26 for the circulating water is provided with the customary inlet and outlet for cooling water. When starting up the system, or at other times when needed, water may be introduced through the pipe 21.

The cooled gas leaving the top of the :nal

Acooler 4 passes through a pipe 30 into the bottom of an absorber 32 for acidic gases. The absorber illustrated is of the two-section type, having a lower section 34 and an upper section 36. Highly actifledsolution of sodium phenolate is supplied to the top of the upper section 36 of the absorber through a pipe 38, and passes downwardly through said upper section and into the lower section 34. In the lower section, it combines with a further quantity of actied solution supplied through the pipe 40, and the combined solution removes the acidic constituents from the gas'. Purified gas is discharged from the absorber through an outlet pipe 42 to the benzol scrubbers or the gas distributing system.

The phenolate solution containing absorbed HzS, HCN and CO2 is collected in -a suitable sump or storage tank 44 at the bottom of the absorber, from which it is delivered by a pump 46 through a heat exchanger 48 and a pipe 58 to the top of an actiiler 52. In this actifier, the solution is heated by steam, which may suitably be supplied t'o coils 54 at the bottomof the actiiier tower, and the HzS, CO2 and HCN are driven out of the .highly actifled solutio solution as previously described, passing from the actiiler through a pipe 56 to a dephlegmator 58, which condenses and returns to thev actier any water and phenol carried out `with the acid gases. The HCN, HzS and CO: then pass on through a pipe 60 for further treatment.

Partially or substantially completely actified solution may be withdrawn from an intermediate portion of the actier through a pipe 62, and 'returned as by pump 84 through pipe 06, and the heat exchanger 48 to a pipe 68, and a cooler 'l0 and pipe 12 into pipe 40, which discharges it into the vlower section of the absorber 32. The remainder of the solution passes downwardly through the actier, 'and is discharged through pipe 14 and p p 18, which delivers the through a pipe 18 to the heat exchanger 48, or another heat exchanger, then through a pipe and a cooler 82 into pipe 38, which returns it to the top of the absorber. 20 In the absorber, this solution takes up phenol vapors and acidic gases, as previously described.

The acidic gases discharged from the dephlegmator58 through the pipe 60 are compressed as by a compressor 84, which discharges them into a scrubber 86, to which cold Water is supplied, as by a pipe 88. The conditions of temperature and pressure are so maintained that all of the HCN and a minimum of HzS and CO2 are absorbed in the water. The HzS and CO2 which are not absorbed, are discharged from the scrubber through a pipe 80, by which they may be delivered to a sulfuric acid plant or other point of disposal.

The aqueous liquid containing the HCN and other gases absorbed in the pressure scrubber 86 is withdrawn by a pump 92 and delivered through a pipe 94 to' another scrubber 86, although part of this solution may be recirculated or by-passed from pipe 94 through a pipe 98 into the pressure scrubber 86, if desired. In the scrubber 96, the

HCN solution containing COz andl HzS is scrubbed, preferably by a counterflow of purified coke oven gas which may conveniently be withfdrawn from the purified gas line 42 through a pipe |00 by the blower |02,- and delivered into the bottom of this scrubber 86. The gas passing through thesolution may be readily controlled to eliminate al1 the I-IzS and CO2 from the solution, usually taking with it a small proportion ofthe HCN. 'I'his gas is discharged from the scrubber 86 through a pipe |04, and'rnay con'- veniently -be returned to the pipe 30, and thence to the absorber 32, where the acid gases are again taken up and -the purified gas is sent forward. 'Ihere is thus no loss of acid gases from the system.

The Purified' HCN solution is withdrawn from the bottom of scrubber 98 through a'pipe |08 by a vpump |08, which delivers it through a pipe ||0 to another container I I2, where the HCN is liberated by a stronger acid such as H2804 supplied through a pipe I I4. Upon mixture with the acid, the HCN is readily liberated from the solution, and is withdrawn from the tank H2 through a pipe II6, connected to the intake of a vacuum pump |.|8. The acidiiied solution free of HCN is discharged-from tank ||2 through an outlet |20, for any desired disposal.

The gaseous HCN is delivered by the vacuum 70 pump ||8 to a reaction tank |22, where it is contion is supplied near the top through a pipe |24. 18

The caustic `solution ows downwardly through the tower, reacting with the HCN to form NaCN in solution, and any of the HCN which is not absorbed may be discharged from the tower through a pipe |26, and returned to the inlet of the vacuum pump H8, and thus recirculated until it is absorbed. The sodium cyanide solution is withdrawn from the tower through a pipe |28, and may be Worked up into some other compound, or concentrated by evaporation, or recirculated through pipe |24 if desired.

It is thus a simple and economical matter, in accordance with my invention, to recover separately, for utilization or other disposal, substantially the entire amount of acidic gases present in coke oven gas and the like. Optimum conditions for maximum recovery and separation 1 of the acidic gases vary somewhat with the nature of the original gas, and with the original proportions of the acidic constituents therein. In general, however, satisfactory results are obtained by scrub` bing the mixture of HzS, HCN and CO2 at a pressure of 50 to 100 pounds per square inch. This forms a solution in whichA the ratio of HCN to the other gases is much higher than in the gas from the actiier. The purifiedcoke oven gas or other inert gas employed to remove the HzS and CO2 from this solution need only be suflicient to give a concentration of about 600 to '700 grains of HzS per 100 cubic feet in the gas discharged from this tower to an earlier stage in the system. The cyanide solution remaining after this treatment may advantageously have a concentration of about l to 2 per cent of HCN. It should be understood, howevenlthat these data are given by way of illustration only, and not by way of limitation.

What I claim is:

l. The method of recovering HCN` from a. mixture oi acidic gases constituting the eiiluent from the actif-ler of a hot-actication gas purifying system, said mixture containing HzS as a principal constituent and a smaller quantity oi'l HCN, and said purifying system including an'absorber in which the said acidic gases are removed from coke oven gas, which comprises scrubbing the acidic gaseous eilluent from said actifler with water under conditions oi' temperature and pressure which favor the absorption of HCN and thus obtaining an aqueous solution containing substantially all the HCN in said gaseous mixture, contacting said solution with a ilow of coke oven gas puried from acidic constituents in the said ab- Ivsorber and thereby removing. from the solution absorbed constituents of said gaseous mixture other than HCN, returning the said gas to the Y said absorber with the constituents which it removes from said aqueous solution of HCN, and recovering HCN from the solution.

2. The method of purifying cokel oven gas, which comprises renoving ammonia from the gas, cooling the substantially ammonia-free gas, absorbing acidic constituents including HzS and HCN from the cooled gas with a solution comprising sodium phenolate, recirculating the sodium phenolate solution through an actifying stage wherein it is heated to drive ofl' said acidic constituents and a stage wherein it is cooled and the absorbing stage wherein said acidic constituents4 are removed from gas thereby, withdrawing from said actifying `stage a mixture of acidicgases including HzS and HCN removed from the coke oven gas in said absorbing stage, contacting said mixture of acidic gases with water to form a solution containingsubstantially all of the HCN and some of the other acidic gases of said mixture, contacting said solution with a stream of purified coke oven gas freed from acidic constituents in said absorbing stage to remove said other acidic gases from the solution and thus preparing a substantially pure aqueous solution of HCN, returning the .resulting mixture of coke oven gas and acidic gases to the said absorbing stage for separation of the acidic gases therefrom, and recovering substantially pure HCN from said aqueous solution thereof.

3. The method of claim 2, wherein said cooling of the ammonia-free gas is eiected by direct oontaet with cooling water, and wherein said cooling water is recirculated through a stream of said gas and through a stage in which it is cooled without loss of constituents absorbed from said gas.

.4. The method of claim 2, wherein said cooling of the ammonia-free gas is effected by direct contact with recirculated cooling water, and wherein condensate formed from the gas is withdrawn from the recirculatory cooling Water system and distilled and gaseous constituents thereof-are returned to a stream of said coke oven gas for recovery thereof.

5. The method of claim 2, wherein said mixture of acidic gases is compressed and scrubbed with cold water to form said solution containing sub-, stantially all of the HCN removed from the said coke oven gas.

FREDRICK D. SCHREIBER. 

